The present application relates to a testing method for investigating in a noncontact and nondestructive manner the film property of an amorphous or polycrystalline oxide semiconductor layer useful as an active layer of a thin film transistor or the like, and to a method for making an amorphous or polycrystalline oxide semiconductor layer utilizing the testing method.
Field effect transistors (FETs) designed as thin film transistors (TFTs) are widely used as pixel transistors in electronic circuits, in particular, active matrix circuits of image display devices. Currently available TFTs generally use amorphous silicon or polycrystalline silicon as a semiconductor material constituting active layers, and glass substrates as substrates.
However, glass substrates are heavy, readily break upon impact, and have no flexibility. Thus, research and development on light-weight, flexible plastic substrates that are not easily breakable and can replace glass substrates is now under way. Since a high-temperature heat treatment process is required for making silicon thin films, it is difficult to form silicon thin films on plastic substrates having low heat resistance. This has led to a focus on amorphous or polycrystalline metal oxide semiconductor materials that can be formed into films at low temperature and serve as a semiconductor material that replaces silicon.
For example, Japanese Unexamined Patent Application Publication No. 2006-165529 ('529 document) (pp. 6 to 9, 16, 21, and 222, and FIG. 3) proposes an amorphous oxide based on In—Ga—Zn—O or the like containing indium, gallium, and zinc as main constituent elements. This amorphous oxide is characterized in that it includes crystallites, has a composition that varies in the layer thickness direction, or contains at least one predetermined element, and in that it has an electron carrier density less than 1018/cm3 or shows a tendency that the electron mobility increases with the electron carrier density. This document provides the following description.
When an amorphous oxide represented by a compositional formula, ZnxGayInzOx+3y/2+3z/2 or the like is formed into films by regular sputtering techniques, oxygen defects are readily formed and a large number of carrier electrons are generated, thereby giving an electron carrier density of 1018/cm3 or more and an electrical conductivity of 10 S/cm or more. This oxide is a useful conductor but is rarely used to make normally off TFTs because when this oxide is used in active layers of TFTs, a large electrical current flows between the source electrode and the drain electrode in the absence of a gate voltage. Moreover, it is also difficult to increase the ON/OFF ratio.
However, in the case where an In—Ga—Zn—O-based amorphous oxide is formed into films by vapor deposition techniques such as pulsed laser deposition or sputtering using a target composed of a polycrystalline sinter represented by a compositional formula, InGaO3(ZnO)m (m is a natural number less than 6), the number of oxygen defects can be reduced by maintaining the oxygen partial pressure in the deposition atmosphere to a particular level or higher, and as a result, the electron carrier density can be suppressed to less than 1018/cm3. The electron mobility observed was more than 1 cm2/(V·sec), leading to the finding of a unique characteristic that the electron mobility increases with the number of conduction electrons. A flexible TFT that has desired properties and is transparent under visible light can be made if its active layer can be formed by using this amorphous oxide.
The amount of oxygen deficiency in the amorphous oxide can also be controlled by processing the oxide film in an oxygen-containing atmosphere after the deposition. During this process, in order to effectively control the amount of oxygen deficiency, the temperature of the oxygen-containing atmosphere is controlled to preferably 0° C. to 300° C., more preferably 25° C. to 250° C., and most preferably 100° C. to 200° C.
FIG. 6 is a graph disclosed in the '529 document showing the relationship between the oxygen partial pressure in the atmosphere and the electrical conductivity of the oxide semiconductor layer in the case where an In—Ga—Zn—O amorphous oxide semiconductor layer is formed by sputtering. FIG. 6 shows that assuming that the adequate electrical conductivity is 10−6 to 10 S/cm, the oxygen partial pressure is desirably controlled within a narrow range of 3×10−2 to 5×10−2 Pa.
When the oxide semiconductor layer is used as an active layer of a FET, the carrier density in the oxide semiconductor layer is a parameter crucial for determining the element characteristics. However, as shown in FIG. 6, the carrier density in the oxide semiconductor layer is highly sensitive to the deposition conditions. Moreover, since the carrier density in the oxide semiconductor layer is unstable, it changes by a magnitude of several orders depending on the atmosphere and the temperature of the steps and chemical treatment subsequent to the deposition process. Thus, the carrier density is adjusted by an annealing process under a controlled atmosphere.
If the carrier density of the oxide semiconductor layer during deposition, during annealing, or upon completion of fabrication can be quickly checked by nondestructive testing, oxide semiconductor layers and eventually semiconductor elements such as FETs can be fabricated in a high production yield.
In general, the carrier density of a semiconductor layer is determined by measurement that utilizes the Hall effect. However, the Hall effect measurement is not suitable for quickly determining the carrier density nondestructively since a Hall element for the Hall effect measurement is prepared.
On the other hand, Japanese Unexamined Patent Application Publication No. 2000-28518 ('518 document) (pp. 2-4 and FIG. 1) reports an example of measuring the carrier density by a photoluminescence technique. FIG. 7 is a spectrum of the photoluminescence from the InGaAs epitaxial film disclosed in the '518 document. FIG. 7 shows that the spectrum of the photoluminescence emitted from single crystals has a peak at a wavelength corresponding to the bandgap energy. The carrier density can be derived by analyzing the shape of the peak. However, photoluminescence is rarely observed from amorphous or polycrystalline oxide semiconductor layers at a wavelength corresponding to the bandgap energy. Thus, the technique disclosed in the '518 document is not applicable.